Fix sftp users ssh resource, add custom domain tag and more improvments

[Hax7]

what

• Fixed IAM policies and roles names to have user name because fixed names will prevent policy/role creation since it will be exist
• Fixed users ssh resource to have for_each and point to the correct public key and user name
• Add setting custom domain for SFTP server using transfer:customHostname tag
• deprecate var.enable_sftp in favour of var.enabled

why

• General improvements and cleaner usage

[themaniskshah]

LGTM!

[vaibhav7797]

LGTM!

[Hax7]

I think I still need to update the examples and add details about how to use directory mapping for restricted home directory, it's not clear in the docs and i had to dig in the code for it